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A mid-infrared (mid-IR)-focusing subwavelength grating (SWG) coupler and suspended membrane waveguide (SMW) on a silicon-on-insulator wafer are studied. For a transverse-electric mode uniform SWG, finite-difference time-domain simulation predicts 44.2% coupling efficiency with 1 dB bandwidth of about 220 nm and backreflection of 0.78% at 2.75 μm. Then the uniform SWG is curved to a focusing SWG using a phase-matching formula. The SMWs are analyzed by the finite element method and fabricated. An Er3+-Pr3+ co-doped mid-IR fiber laser is used for device characterization. The fabricated mid-IR SWG coupler has 24.7% coupling efficiency.

We propose and demonstrate broadband focusing grating couplers for suspended-membrane waveguides on silicon-on-insulator both in near-infrared (near-IR) and in mid-IR wavelength range. Finite-difference time-domain simulation predicts ∼100  nm 3 dB bandwidth with -1.7  dB coupling efficiency for an apodized grating in near-IR. -3.5  dB maximum coupling efficiency and ∼90  nm 3 dB bandwidth are realized experimentally. In mid-IR, -5.5  dB maximum coupling efficiency from a uniform focusing grating is measured at 2.75 μm, and ∼500  nm 3 dB bandwidth is predicted theoretically.

We demonstrate an apodized focusing subwavelength grating (SWG) for suspended membrane waveguides on silicon-on-insulator. Finite-difference time-domain simulation predicts −1.7 dB coupling efficiency and a 3 dB bandwidth of ∼50 nm for the transverse-magnetic mode apodized SWG, which has 98% field overlap with propagation mode in the single mode fiber. A modified phase matching formula is proposed to design the focusing apodized SWG. Better than −3.0 dB coupling efficiency and a 3 dB optical bandwidth of ∼50 nm is demonstrated experimentally.